3 days late, negative test. ???

My period is 3 days late, which is strange for me. Being a day late isn't usual, but three? Obviously I thought I was pregnant so I bought a test and it read negative (no line). I'm not one hundred percent sure I did it right (didn't hold it in the stream for 5 full seconds, but I'm pretty sure it's a true negative and I'm not pregnant. If my period is that far late and I'm not pregnant, should I see my doctor? Is this a valid concern? No, I haven't lost tons of weight or anything that could offset it, only difference is I started a new antidepressant about two weeks ago. Could that affect it? Am I being crazy?